I was studing a bit the problem, and I realized that into the dbispconfig database there is a table names mail_user, where there is a quota field.

So I think that the logic way to solve the problem is realize a script that check that quota field for every email and, when this quota is 80% that the space assigned to the customers, just send an email to them saying that the space is almost finished.
Do you think it is difficult to realize?
Any idea?

At the moment I've 2 problems:
1) I don't know where to find the $ROUNDCUBE_USER, means the email of the user that at that time is using roundcube
2)I don't know where this slider is setting his variables releated with the status (0-10)

Hey,
I've found a script that check the quota of each email and, if it's more that 90%, it will send an email to me and the customer.But I think that having this funcion integrated in ISPConfig could be just great...
Any idea?

Here is a PHP script I wrote some time ago that uses phpmail to send the notification. For those who are more conformable with PHP. You probably want to change the SQL statement to select mailboxes that are active, or only select whose with a quota.

Not a fast script as it uses the "du" command line to get the size of the mailbox folder, so probably not great if you have thousands of email boxes to check